Method of treating steel

ABSTRACT

STEEL STOCKS ARE IMMERSED IN CERTAIN SOLUTIONS AND THE SOLUTIONS ARE HEATED AND PRESSURE IS APPLIED THERETO TO FORM A CORROSION RESISTANT AND HEAT RESISTANT PROTECTIVE FILM ON THE STEEL.

United States Patent Int. Cl. czsr 7/24 U.S. Cl. 1486.24 8 Claims ABSTRACT OF THE DISCLOSURE Steel stocks are immersed in certain solutions and the solutions are heated and pressure is applied thereto to form a corrosion resistant and heat resistant protective film on the steel.

RELATED APPLICATION This application is a continuation-in-part of application Ser. No. 689,795, filed Dec. 12, 1967, now abandoned.

FIELD OF INVENTION This invention relates to a method of treating steel stocks under pressure and has an object to provide protective coatings having excellent corrosion resistance comparative to that of zinc plated sheet when immersed in water.

BACKGROUND OF INVENTION Many methods of treating steel stocks have been used to impart corrosion resistance to the surface thereof. For example, steel sheets have been electrolyzed in an electrolyte consisting of a chromate or a phosphate, respectively, incorporated with various organic chemical additives and then baked. Further steel sheets are plated with metals. However, the process steps of these prior treatments are very complicated, especially when products which are rust proof over an extended period of time are required. To meet this requirement, it is also necessary to increase the thickness of the plating, thus increasing the cost of the products. More specifically, when it is desired. to obtain sufficient corrosion resistance by formation treatment comparable to that of metal plating the manufacuring cost becomes prohibitive. Steel sheets subjected to a simple formation treatment to an extent to provide temporary rust proofness for the purpose of warehouse storage can prevent formation of red rust only for about one week. Even zinc plated steel sheets form oxide coatings thus greatly decreasing their commercial value.

' We have succeeded to provide products having excellent rust'proofness, especially in a high humidity environment over a long time, and can manifest excellent corrosion resistance in running Water comparative to that of zinc plated sheets by a relatively simple method of formation treatment. Products according to this invention have Wider applications.

SUMMARY OF THE INVENTION In accordance with this invention, there is provided a method for protecting the surface of steel stock against corrosion. The said surface is immersed in an aqueous solution comprising a calcium or magnesium molybdate solution, an alkaline solution of magnesium oxide, a sodium sulfide solution containing a calcium or magnesium molybdate, or a sodium sulfide solution. Steel sheets are immersed in one of said solutions contained in an autoclave maintained at a temperature of from about 240 C.

to about 400 C. and at a pressure suificient to maintain the solution and the metal surface at said temperature, preferably from about 40 to about 300 kilograms per square centimeter (kg./cm. The sheets can be immersed thereafter in chromic acid, if required.

SPECIFIC EMBODIMENTS OF THE INVENTION More particularly, when a molybdate (M.MoO M being Mg or Ca) is utilized as the treating solution, corrosion resistant films are obtained which are identified by X-ray or electron beam diffraction analysis as combinations of FeO, 'y-Fe O Fe O M00 FeMoO and compounds of alkaline earth metal M. Where steel sheets are treated in an alkaline solution containing magnesium oxide under pressure and at an elevated temperature, dense corrosion resistant films are formed consisting of iron oxides and magnesium compounds formed by the reaction between the solution and the surface of steel sheets. When treated with sodium sulfide solution, FeO, -Fe O Fe O Fe -xS, FeS etc. are formed on the surface of steel sheets, these iron compounds cooperating to provide excellent corrosion resistance. Since the surface condition of the films varies dependent upon the pressure and since the films are porous, steel sheets can be subjected to an after treatment by dipping them in a chromic acid solution. Sodium sulfide solution incorporated with a small amount of M.Mo0 results in oxides or sulfides similar to those formed by said sodium sulfide solution in addition to compounds of molybdenum and alkaline earth metal, thus further increasing corrosion resistant properties. In each of these cases, it is advantageous to after-treat the products with a chromic acid solution.

The following specific examples are given by way of illustration, and are not to be construed as limiting in any way the scope and spirit of the invention.

Example 1 TABLE 1 Sample numbers Conditions 1 2 3 4 Temperature, C 240 285 325 385 Pressure 38 70 300 TESTS IN ACIDIC SOLUTIONS Test pieces shown in Table 1 were immersed in solutions of various pH values of from 2 to 7. It was noted visually that in a strongly acidic solution of pH 2 the surface film of each of test pieces 1-5 was dissolved in about 24 hours. However, after this time, weight loss of Samples 2-5 thereof was smaller than that of untreated cold rolled steel sheet (Sample 1) which demonstrated that corrosion resistance was influenced substantially by pressure and heat. These test results are shown in Table 2 below.

Patented Oct. 31, 1972 Surface conditions of respective samples immersed for 40 days in solutions of pH 4 to 7 are shown in Table 3.

Norn.-A=Suriaee films of the samples were perfect; B=About 2 to 3% of the iron substrate was exposed; C=About 3 to of the iron substrate was exposed; D=Only a small portion of the film remained; E= Wholly corroded.

As can be noted from Tables 2 and 3, for the same solution, corrosion resistance increased with pressure. Particularly, as shown in Table 2, when immersed in a strong acidic solution surface films have completely disappeared (when viewed by the naked eye) in about 24 hours, weight loss by dissolution showed low values for long periods thereafter. Further, treatment at higher pressures resulted in lower weight losses.

Example 2 This example shows the results of treating cold rolled steel sheets in accordance with thisinvention followed by an after treatment with an aqueouschromic acid solution.

(1) The steel sheets were treated in a 0.25 mol aqueous solution (Na,S:1.95 wt. percent) of sodium sulfide at a temperature of 325 C. and under a pressure of 1 10 atmospheres pressure for 24 hours, and then treated in a chromic acid solution of a concentration of 0.3 to 2.0%.

(2) Corrosion tests were performed by immersing test pieces in a running water tank of the dimension 100 cm. x 100 cm. x 50 cm. through which water was circulated at a rate of 125 cc./min. to determine the weight loss by corrosion.

(3) Results: As shown in Table 4 following, the corrosion weight losses of test pieces treated in accordance with this invention are far smaller when compared with untreated cold rolled steel sheets, but weight losses were dependent upon the concentration of chromic acid employed in the after'treatment.

TABLE 4 Weight losses by corrosion (mgJcmJ) Immersion days Example 3 This example shows comparison of results obtained by treatingwith above described various solutions.

Conditions of treatments: Each sample was treated in 4 a particular solution contained in an autoclave maintained at a temperature of 240 C. under from 39 to 40 atmospheres pressure.

(1) Table 5 below shows the conditions of surface corrosion after standing for 30 days in a warehouse.

TABLE 5 Sample numbers: Conditions of corrosion Remarks:

Sample No. 1 is a steel sheet treated with an aqueous magnesium molybdate solution (MgMoO 4.6 wt. percent).

Sample No. 2 is a steel sheet treated withan aqueous calcium molybdate solution (CaMoO 3.2 wt. percent).

Samples No. 3 is a steel sheet treated with an alkaline aqueoussolution of magnesium oxide (NaOI-I: 1.85 wt. percent; MgO: 1.85 wt. percent).

Sample No. 4 is a steel sheet treated with a chromic acid solution (CrO 2.0 wt. percent).

Sample No. 5 is a steel sheet treated with an aqueous sodium sulfide solution containing a small quantity of magnesium molydbdate (MgMo0 0.5 wt. percent).

Sample No. 6 is a steel sheet treated with an aqueous sodium sulfide solution (Na S: 1.95 wt. percent).

Sample No. 7 is an untreated cold rolled steel sheet.

Wet test (98% humidity at 50. C.)

Table 6 below shows conditions of surface corrosion after standing 7 days.

TABLE 6 Wet test Sample numbers: Conditions of corrosion Table 7 below shows the conditions of surface corrosion after immersing for 30 days in running water.

TABLE 7 Running water immersion 'test Sample numbers Conditions of corrosion The results of the above corrosion tests show that Sample No. 1 is the best and the next are Sample-Nos. 3 and 5. Next, in Table 8 Sample No. 3 showed excellent corrosion resistant properties when immersed in sulfuric acid solutions.

Example 4 Cold rolled steel sheet was treated with an aqueous sodium molybdate solution (2.0 wt. percent) or with an aqueous magnesium molybdate solution (1.8 wt. percent) at 280 C. under a pressure of 100 kg./cm. for 24 hours.

The conditions of surface corrosion of the steel sheet treated as described above after it is left in a moisture tank (temperature: 50 C., humidity: 98%) for 14 days are as follows:

TABLE 9 Agent: Conditions of corrosion Sodium molybdate D Magnesium molybdate A Results in Table 9 reveal that treatment with an alkali metal molybdate solution (Na MoO is unsatisfactory, in contrast to treatment with a magnesium molybdate solution.

Thus, this invention provides highly corrosion resistant protective films on steel stocks by immersing them in a solution selected from a molybodate solution, such as a calcium molybdate solution (.l3-6.42% by weight) or a magnesium molybdate solution (0.18-9.21% by weight), a mixture of magnesium oxide solution (0.4-8% by weight) and alkaline solution such as a sodium hydroxide solution (O.4- 8% by weight) or a potassium hydroxide solution (0.56-l1.2% by weight), a sodium sulphide solution (0'.39-7.8% by weight), or a sodium sulphide solution (0.39-7. 8% by weight) incorporating a calcium molybdate solution (0.13-6.42% by weight) or a magnesium molybdate solution (0.l8-9.4% by weight), at an elevated temperature and under a high pressure. Further, by subjecting the formed films to an after treatment with chromic acid, their corrosion resistant properties can be further increased. When combined with various types of paint coatings, it is possible to provide corrosion proof metal articles without relying upon plating processes. Further, these films have high heat resistant and wear resistant properties.

We claim:

1. Method for protecting the surface of steel stock against corrosion, which comprises immersing said surface in an aqueous solution selected from the group consisting of a solution consisting essentially of from 0.13 to 6.42 percent by weight of calcium molybdate and water, a solution consisting essentially of from 0.18 to 9.21 percent by weight of magnesium molybdate and Water, an alkaline solution consisting essentially of 0.4 to 8 percent by weight of magnesium oxide, an alkali and Water, and a solution consisting essentially of 0.39 to 7.8 percent by weight of sodium sulfide, a calcium or magnesium molybdate and water wherein there is 0.13 to 6.42 percent by weight of calcium molybdate or 0.18

to 9.21 percent by weight of magnesium molybdate, anda solution consisting essentially of sodium sulfide and water, at a temperature of from about 240 C. to about 400 C. and a pressure of from about 40 to about 300 kg./cm.

2. Method of claim 1, wherein the aqueous solution is a sodium sulfide solution containing magnesium molybdate.

3. Method of claim 1, wherein the aqueous solution is a sodium sulfide solution.

4. Method of claim 1, wherein the aqueous solution is a magnesium molybdate solution.

5. Method of claim 1, wherein the aqueous solution is a calcium molybdate solution.

6. Method of claim 1, wherein the aqueous solution is an alkaline solution of magnesium oxide.

7. Method of claim 1, wherein the steel stock is thereafter treated with an aqueous chrornic acid solution.

8. Method of claim 1, wherein the aqueous solution is a sodium. sulfide solution containing calcium molybdate.

References Cited UNITED STATES PATENTS 1,723,067 8/1929 Pacz l486.14 3,272,665 9/l966 Romans 1483l.5 X 3,248,249 4/ 1966 Collins 1486-.16 X 3,272,66-3 9/1966' Kronstein 1486.16 2,557,509 6/ 1951 Miller l486.24 2,829,995 4/1958 Newton l486.24 2,313,925 3/1943 Curtin l486.24 2,755,170 7/ 1956 Stubblefield l486.l4 X

RALPH S. KENDALL, Primary Examiner C. WESTON, Assistant Examiner U.S. Cl. X.R. 1486. 14 

